Conformation of Cyclohexane

Cyclohexane is an alicyclic hydrocarbon comprising a ring of six carbon atoms that plays a crucial role in organic chemistry. Cyclohexanes exhibit great stability due to their ability to adapt to different conformations, minimising strain energy.  Understanding the conformation of cyclohexane and its derivatives is essential in chemistry.

1.0Understanding the Conformation of Cyclohexane

Cyclohexanes are non-planar molecules, as a completely planar structure would result in significant angle strain and torsional strain. To bring down these issues, the cyclohexane molecules adopt conformations that reduce the strain energy and increase stability.

• Chair Conformation
• Boat Conformation
• Twist-Boat Conformation
• Half-Chair Conformation

Each conformation has its own energy profile and stability, which is crucial for understanding the conformation of cyclohexane stability order.

2.0Chair Conformation: The Most Stable Form

The most stable conformation of a cyclohexane is the chair conformation. It is named as such because its structure resembles a chair. It eliminates angle strain as the bond angles are close to the tetrahedral 109.5°. It also eliminates torsional strain due to its staggered C-H bonds. In this conformation:

• Six hydrogen atoms are in axial positions aligned parallel to the ring's imaginary axis.
• Six hydrogen atoms are in equatorial positions that are aligned roughly in the plane of the ring.
• The axial and equatorial positions alternate around the ring.

Chair Flip: Axial-Equatorial Interconversion

The chair flip is a dynamic process where the molecules rapidly interconvert between two stable chair conformations, switching axial and equatorial substituents. The flipping procedure is crucial to understanding the reactivity of cyclohexane and its derivatives. 

3.0Boat and Twist-Boat Conformations

The cyclohexane boat conformation is less stable than the chair conformation. It is a high-energy structure due to steric strain and eclipsed hydrogen interactions. In this conformation:

• The molecule resembles a boat with two peaks.
• Eclipsing interactions cause torsional strain.
• Flagpole interactions occur due to hydrogen atoms at the bow and stern coming too close together.

4.0Twist-Boat Conformation

To relieve strain, the boat conformation distorts to twist-boat conformation, reducing eclipsing interactions and steric hindrance. While it is more stable than the boat form, it is still less stable than the chair conformation. 

5.0Half-Chair Conformation: The Transition State

The half-chair conformation of cyclohexane is a key transitional state during the interconversion between the chair and twist-boat conformations. It represents a high-energy and unstable state where five carbon atoms are coplanar, and one is plucked out of the plane. It is rarely observed in isolated states. 

6.0Energy Diagram of Cyclohexane Conformations

To visualise the stability of different conformations, the conformation of cyclohexane energy diagram is essential. The energy profile follows this order:

1. Chair conformation has the lowest energy. It is also the most stable.
2. Twist-boat conformation has a slightly higher energy than a chair.
3. Boat conformation has an even higher energy due to steric strain.
4. Half-chair conformation has the highest energy, and it is observed in the transition state.

7.0Energy Profile Diagram

The conformation of cyclohexane and the energy profile diagram illustrates the energy changes as the molecule undergoes a chair flip:

This energy variation explains why cyclohexanes predominantly exist in chair form under normal conditions. 

8.0Energy and Stability of Cyclohexane Conformations

Refer to the table below to understand the energy of cyclohexane conformations and the conformation of cyclohexane stability order.

9.0Rigid Conformation of Cyclohexane and Its Derivatives

Certain cyclohexane derivatives contain bulky substituents that restrict ring flipping, leading to a rigid conformation of cyclohexane. For example:

• Tert-butylcyclohexane: The bulky tert-butyl group locks the ring in a single chair conformation with the bulky group in the equatorial position.
• Bicyclo[2]octane and other fused rings: These structures prevent chair flipping due to additional ring strain.